1. Field of the Invention
This is directed to a star-shaped, assymetric polymer. More particularly, this invention is directed to a star-shaped block copolymer having monoalkenyl arene-conjugated diene copolymer arms and conjugated diene homopolymer arms. Further, adhesive compositions containing such block copolymers are also part of the invention.
2. Description of the Prior Art
Block copolymers have been increasingly employed in adhesive compositions primarily because of their high cohesive strengths and their ability to "cross-link" without a chemical vulcanization step. These block copolymers, such as those described in Harlan, U.S. Pat. No. 3,239,478, are primarily either linear or radial styrene-butadiene or styrene-isoprene block copolymers. The linear copolymers typically have the structure styrene-butadiene-styrene or styrene-isoprene-styrene. The high cohesive strength of these styrene-diene block copolymers is primarily attributed to their network structure resulting from their well-known domain formation.
However, this very high cohesive strength is often a detrimental quality in certain applications. For example, it is known that in adhesives based on natural rubber or SBR, aggressive tack is inversely related to cohesive strength. That is, as the rubber is crosslinked to a higher degree, the cohesive strength increases but the aggressive tack decreases. Previous patents disclose adhesive compositions containing block copolymers which impart very high cohesive strength to the adhesive. In formulations containing only block copolymer and tackifying resin, aggressive tack is marginal and oil is normally required in the formulation to improve aggressive tack. If some way were found to reduce the cohesive strength of the block copolymer used in an adhesive formulation, substantially improved aggressive tack could be obtained, even in formulations which do not contain oil.
One way to reduce cohesive strength is by reducing coupling yield of the SIS polymer or by blending the polymer with a high molecular weight SI diblock copolymer. Such mechanisms are disclosed in U.S. Pat. No. 4,096,203. These approaches should indeed lower cohesive strength and give less elasticity and better conformability. However, they also cause a reduction in the upper service temperature of the adhesive. In pressure sensitive adhesive (PSA) label applications, in which service temperature requirements are modest, such low coupled versions have had some success. However, in PSA tapes which must bear significant load at 60.degree. to 70.degree. C., such block copolymers have poured to be of little use.
A new block copolymer has been found which has a unique set of property advantages.